In a liquid developing agent for electrostatic photography, a printing ink and a coating composition, it is required that a pigment or dye, or a colorant as formed by coating a pigment or dye with a fixing resin, or a fixing resin is stably dispersed in the form of fine grains. In a liquid developing agent for electrostatic photography, in particular, since a colorant which is dispersed in a high electric-resistance non-aqueous solvent and which is called a toner controls the image quality of the image as developed, the toner is required to have a good dispersing property.
In an electrostatic photographic process, in general, the means of developing a formed electrostatic latent image to the corresponding visible image includes a dry development method and a liquid development method. The latter liquid development method is superior to the former dry development method, since the toner grains may be finer and therefore fine images and halftone images may be faithfully reproduced.
In general, a liquid developing agent for electrostatic photography comprises a colorant such as various pigments or dyes, typically carbon black, a coating agent to adsorb onto the colorant or to coat the same so as to adjust the charge of the colorant or to impart a fixing property thereto, a dispersing agent to impart a dispersing property to the toner grains, a charge-adjusting agent to adjust the polarity of the toner grains and the amount of the charge thereof, and a high electric-resistance non-aqueous solvent carrier liquid having a dielectric constant of 3 or less and a volume specific resistance of 10.sup.9 .OMEGA.cm or more. Pigments and dyes are unnecessary in a printing plate or the like where the toner image is required to have only an inkadhesive property and in the case where only a resist property to an etching solution is required in processing a printing plate. A liquid developing agent, a printing ink and a coating composition are generally prepared, for example, by the methods mentioned below. A pigment or dye, a fixing resin, or a pigment or dye as coated with a fixing resin is directly, or after being dry-powdered, dispersed along with a dispersing agent-containing high electric-resistance non-aqueous solvent, by the use of a wet dispersing machine such as a ball mill, paint shaker or sand mill, to a desired grain size to obtain a thick dispersion. The thus obtained thick dispersion may be used directly or after dilution with a solvent. In the case of a liquid developing agent, the thick dispersion is added to a charge-adjusting agent-containing carrier liquid to give a liquid developing agent having either a positive or negative polarity.
As described above, a non-aqueous dispersing agent for wet dispersion in the present invention means a composition which is used to finely disperse a pigment or dye in a non-aqueous solvent by a wet-dispersing machine such as a ball mill, a paint shaker and a sand mill.
As the dispersing agent for use in a non-aqueous liquid, hereafter referred to as the non-aqueous dispersing agent, to be used therein, there are generally mentioned fats and oils which are soluble in a carrier liquid; long-chain carboxylic acids such as oleic acid; oil-modified alkyd resins; esters of abietic acid; as well as homo-polymers of long-chain alkyl acrylates or methacrylates such as 2-ethylhexyl (meth)acrylate or stearyl (meth)-acrylate or random copolymers of such (meth)acrylates with styrene or vinyl toluene or with methyl (meth)-acrylate or ethyl (meth)acrylate.
However, such conventional dispersing agents could attain a dispersing property and a dispersion stability after dispersion to some degree when a pigment or dye is dispersed singly with the agent, but they display an extremely lowered dispersing property when a colorant of a pigment or dye as coated with a fixing resin or a fixing resin itself is dispersed with the agent. As the case may be, the resin of the dispersing agent and the resin of the coating agent would often be combined and bonded to each other to form a coagulated product or a solid mass. On the other hand, a liquid developing agent to be used in preparing a printing plate by dissolving the non-image portions with an etching solution is required to have a resist-resisting property and therefore a hardly dispersible fixing resin is generally used in the liquid developing agent. However, the hardly dispersible fixing resin cannot be dispersed with the above-mentioned conventional dispersing agents.
JP-B-59-37826 (the term "JP-B" as used herein means an "examined Japanese patent publication") mentions that a morpholino group-having vinyl copolymer is useful as a dispersing agent for a liquid developing agent for electrostatic photography. However, the proposed copolymer is suitable only for dispersion of pigments. The dispersing effect of the copolymer is insufficient for dispersing fixing resin-coated pigments. JP-A-60-10263 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") mentions that a block copolymer comprising styrene and a long-chain alkyl methacrylate is useful as a dispersing agent for pigments. However, it only mentions the dispersing effect of the block copolymer of dispersed pigments alone, as the illustrated block copolymer itself is to have a fixing property. Therefore, it is not clarified in the patent application as to whether or not the block copolymer has an effect of dispersing fixing resin-coated pigments. Additionally, production of the illustrated block copolymer is complicated. JP-A-57-128350 illustrates a graft copolymer composed of a soluble vinyl copolymer moiety and an insoluble vinyl copolymer moiety as grafted via a urethane bond. However, the graft copolymer is disclosed to be useful only as a dispersing agent for pigments or dyes alone. The patent application is silent on the dispersing effect, if any, of the graft copolymer for dispersing a fixing resin-coated colorant.
JP-B-56-10619 and JP-B-60-18985 illustrate liquid developing agents which contain a graft copolymer as obtained by copolymerizing a vinyl monomer and a graft-active point-imparting glycidyl methacrylate to give a stem polymer, esterifying the stem polymer with acrylic acid and then further copolymerizing the thus esterified stem polymer with other vinyl monomer. The graft copolymers as illustrated therein act as toner grains by themselves, and they are mentioned therein to be used along with a pigment to further display a dispersing activity for dispersing the pigment.
JP-B-52-3306 and JP-B-57-12985 illustrate graft copolymers to be obtained by esterifying a copolymer of a vinyl monomer and a glycidyl methacrylate with an unsaturated carboxylic acid and then further copolymerizing the esterified copolymer with one or more vinyl monomers. These patent applications mention that the graft copolymers have a dispersing effect of dispersing pigments but are almost silent on the dispersing effect, if any, of the copolymers of dispersing fixing resin-coated colorants or fixing resins themselves. Additionally, it is known that the graft copolymers thus obtained are often difficult to control, i.e., to prevent crosslinking among themselves to form a gel (JP-B-1-24302, JP-A-58-150970). Further, it is also known that the homopolymers formed often dissolve out into a carrier liquid during storage to thereby lower the dispersion stability and the charge stability (JP-A-59-34540).
In general, graft copolymers are prepared by first polymerizing a stem moiety into which plural graft-active points have been introduced in the molecule, then bonding moieties to be grafts to the active points of the stem moiety and thereafter polymerizing the graft moieties. Since the reaction of forming such graft copolymers is complicated, there are various problems such as control of polymerization is difficult, crosslinking often occurs during the reaction to form gels, homopolymers of the stem moieties are formed and the reproducibility of the reaction is poor. Additionally, it is also known that introduction of a larger amount of graft-active points cause rather noticeable gelling. This is considered to occur because, since a prepolymer into which plural graft-active points have been introduced is a so-called polyfunctional monomer having plural polymerizable positions in one molecule, it is easily gelled by crosslinking reaction. If such graft copolymers are used as a dispersing agent, the reproducibility of dispersion is, as a result, poor because of the above-mentioned problems.
In addition to the above-mentioned substances, a styrene/butadiene copolymer is also known as a dispersing agent. However, a dispersion obtained by the use of the copolymer as a dispersing agent often gives a gelled product after storage for a long period of time because of the double bonds remaining in the butadiene moieties.
Thus, at present, satisfactory techniques of overcoming the above-mentioned problems regarding production of useful dispersing agents which may disperse a pigment as well as a dye, or a colorant as coated with a fixing resin, or a fixing resin itself by a wet-dispersing method to give a complete dispersion which is free from the above-mentioned problems are unknown.